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From: Martin Gregorie <martin@address-in-sig.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Strange Socket problem
Date: Sat, 3 Mar 2012 02:13:19 +0000 (UTC)
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On Fri, 02 Mar 2012 17:18:04 -0800, Knute Johnson wrote:

> On 3/2/2012 4:00 PM, Lew wrote:
>> Knute Johnson wrote:
>>> The volatiles exist because the methods that access them can be called
>>> from other threads. I could have synchronized the start() stop()
>>> methods but not easily the socket variable in the run() method. I
>>> thought it was cleaner to just use volatile.
>>
>> I see a problem right there.
>>
>>> public void disconnect() {
>>> if (isConnected())
>>> if (socket != null)
>>> try {
>>> socket.close();
>>> } catch (IOException ioe) {
>>> ioe.printStackTrace();
>>> }
>>> }
>>
>> Since these are controlled by separate synchronization (different
>> 'volatile' variables) there's a race condition trying to work with both
>> at once.
> 
> I don't think so.
> 
>> Also, 'socket' can become 'null' between the check for not 'null' and
>> the 'close()' call.
> 
> Only if I set it to null and I don't.  That is there so that if the
> Socket doesn't make the first connection when disconnect() is called
> that I won't get a NPE.
> 
>> You need to synchronize with 'synchronized' or other strong mechanism.
> 
> I don't think so and here's why; isConnected only gets modified by one
> thread and read by another, socket is only modified by one thread and
> read by another.  In the disconnect() method, as soon as isConnected()
> is called, isConnected the volatile variable is read and that would make
> socket current even if it weren't volatile which it is but I didn't want
> to rely on side effects in case I changed code somewhere.
> 
> Anyway, disconnect() isn't getting called in this situation so it's not
> causing my problem.
> 
> I really appreciate everybody looking at this.  I've got a couple of
> ideas of where to code some traps and I'll have to put those in one
> night and see what happens.

new Socket() can return null (it says, but not why) but I don't think 
that is happening or you'd have seen an NPE.
 

-- 
martin@   | Martin Gregorie
gregorie. | Essex, UK
org       |